1. Field of the Invention
The present invention relates to a wireless local area network (WLAN) and, more particularly, to a method and apparatus for transmitting a frame in a WLAN system.
2. Related Art
Recently, with the development of an information communication technology, various wireless communication technologies have been developed. Among the wireless communication technologies, a wireless local area network (WLAN) is a technology to wirelessly access to an Internet in a home, a company, or a specific service providing area by using portable terminals such as a personal digital assistant (PDA), a laptop computer, and a portable multimedia player (PMP) based on a wireless frequency technology.
Standards for a WLAN are established by institute of electrical and electronics engineers (IEEE) 802.11. IEEE 802.11a/b provide transmission efficiency of 11 Mbps (IEEE 802.11b) or 54 Mbps (IEEE 802.11a) using a frequency band of 2.4 GHz or 5 GHz. IEEE 802.11g provides transmission efficiency of 54 Mbps by introducing orthogonal frequency division multiplexing (OFDM) technology. IEEE 802.11n provides transmission efficiency of 300 Mbps through four spatial streams by introducing multiple-input and multiple-output (MIMO) technology. IEEE 802.11n provides a channel bandwidth of 40 MHz. In this case, transmission efficiency of a maximum of 600 Mbps may be provided.
As the spread of a WLAN is activated and applications using the WLAN are diversified, recently, there is a need for a new WLAN system for supporting throughput higher than data processing speed supported by IEEE 802.11n. IEEE 802.11ac is the next-generation WLAN system which supports very high throughput (VHT) in a frequency domain of 6 GHz or less. IEEE 802.11ac supports a minimum throughput of 1 Gbps on a plurality of stations (STAs). In link for a single STA, a maximum throughput of at least 500 Mbps can be supported. Furthermore, IEEE 802.11ac can support bandwidths of 80 MHz, contiguous 160 MHz, and non-contiguous 160 MHz and/or higher bandwidths. Furthermore, the maximum 256-quadrature amplitude modulation (QAM) method may be applied. For a higher throughput, multi user (MU)-MIMO technology can be supported. In order to support MU-MIMO technology in IEEE 802.11ac, STAs may have one or more antennas. Today, IEEE 802.11ac supports only downlink (DL) MU-MIMO. The number of STAs to which an access point (AP) can transmit packets at the same time is a maximum of 4. When a maximum number of supportable spatial streams is 8, each STA may use a maximum of four spatial streams.
In a DL MU-MIMO environment, a single AP performs transmission and a plurality of STAs performs reception at the same time. In an uplink (UL) MU-MIMO environment, a plurality of STAs performs transmission at the same time and a single AP performs reception. In order to support UL MU-MIMO, there is a need for a method for selecting STAs that perform transmission at the same time. For example, in UL MU-MIMO, STAs that perform transmission at the same time may be selected in a contention-based manner. STAs having traffic to be transmitted request transmission by transmitting a request to send (RTS) message to an AP during a random access period, and only STAs selected in a contention with other STAs may transmit packets during a data transmission period.
In an UL MU-MIMO environment, there is a need for an efficient method for selecting STAs that will perform transmission at the same time.